CSE 143 : 6 Jan 2000

Update 10 Jan: Sample solutions for in-class exercises are available.

Call by reference

Call by reference allows us to write functions that modify their parameters without using pointers explicitly:

void shuffle(int a, int *b, int &c) { int temp = a; a = *b; b = &temp; c = a + *b + temp; } int main() { int a = 1; int b = 2; int c = 3; shuffle(a, &b, c); cout << a << ',' << b << ',' << c << endl; }

File I/O

You should have covered C-style file I/O at the end of 142. Here's a refresher, plus the new and better C++ way:

/* C way */ #include <stdio.h> int main() { FILE *infile, *outfile; char c; /* Open filehandles */ infile = fopen("in.txt", "r"); outfile = fopen("out.txt", "w"); /* Test if open successful */ if (infile != NULL && outfile != NULL) { /* read until EOF */ while ( fscanf(infile, "%c", &c) != EOF ) { /* writing the character each time */ fprintf(outfile, "%c", c); } } /* Close file handles */ fclose(infile); fclose(outfile); } // C++ way: does NOT do the same thing as the above #include <iostream.h> #include <fstream.h> int main() { // Open file streams. Notice "constructor" syntax ifstream in("in.txt"); ofstream out("out.txt"); char word[200]; if (!in) { cout << "Unable to open input file." << endl; return 1; } if (!out) { cout << "Unable to open output file." << endl; return 1; } // Can you guess what the following does? while (in >> word) out << word << '\n'; }

Review

For the first assignment, we'll be picking up right where 142 left off. You must know C pretty well by now. Here's some refresher material:

Strings

Recall the following C string functions:

• strlen(char *str): returns the length, not including null character
• strcpy(char *target, char *source): copies source to target
• strcmp(char *first, char *second): compares first with second

Remember that C strings are zero-terminated; that is, the end of a C string is marked by the special character '\0'. Can you write a plausible implementation of strlen()? strcpy()?

int strlen(char *str) { } void strcpy(char *target, char *source) { }

Can you rewrite the following C++ program to pick the word in the file in.txt that comes first in alphabetical order, and print it to standard output?

#include <iostream.h> #include <fstream.h> int main() { ifstream in("in.txt"); char word[200]; while (in >> word) { } return 0; }

Structs (old and new)

A struct allows you to define a type with named pieces, sometimes called members or fields.

typedef struct { /* C way */ int id; char name[200] } Student; struct Student { // C++ way int id; char name[200] };

Structs can be nested. Try rewriting the above struct so that the name, instead of being a single char array, is a struct with first and last name members. Then, initialize it to your personal data.

struct Student{ int id; }; /* Declaration; initialize this: */ Student s =

How would you print out the name and ID with a comma between them?

cout <<

Optional homework: Write a program that:

• Reads in a set of 10 student names (first and last) from a file
• Assigns each student a unique integer ID
• Prints out student whose last name comes first in alphabetical order

All the pieces, except arrays of structs, are in the notes above. It should take you about an hour at most. I will post a solution on my web page in a couple of days. (Update: solutions are available.)